Film forming method and film forming apparatus of metal plating film

ABSTRACT

A film forming method and a film forming apparatus of a metal plating film allowing suppressing damage of a porous film. A metal plating film on a surface of a metal substrate by solid substitution-type electroless plating method. The film forming method includes preparing the film forming apparatus that includes at least a bottom wall and a sidewall surrounding the bottom wall and that is provided with a housing space, the metal substrate disposed on the bottom surface inside the housing, the porous film disposed on the surface of the metal substrate, and an electroless plating solution housed in the housing space; and using the film forming apparatus, reducing metal ions derived from the electroless plating solution contained in the porous film, and depositing the metal ions on the surface of the metal substrate to form the metal plating film on the surface of the metal substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Japanese patent application JP 2020-174888 filed on Oct. 16, 2020, the entire content of which is hereby incorporated by reference into this application.

BACKGROUND Technical Field

The present disclosure relates to a film forming method and a film forming apparatus of a metal plating film that form a metal plating film on a surface of a metal substrate by solid substitution-type electroless plating method.

Description of Related Art

Recently, a method that forms a metal plating film on a surface of an object to be plated, such as a metal substrate and a wiring of an electronic industry component, has a problem in use of a large amount of a plating solution and a waste liquid of the plating solution. In view of this, from aspects of a manufacturing cost and an environmental load, solid phase methods like a Solid Electro Deposition (SED) method and a Solid Electroless Deposition (SELD) method have been used.

The solid electrolyte deposition method is a method that disposes a porous film, such as a solid electrolyte membrane, between an anode and a substrate serving as a cathode, brings the porous film into contact with the surface of the substrate, applies a voltage between the anode and the substrate, and deposits metal ions contained in the porous film on the surface of the substrate to form a metal plating film made of a metal on the surface of the substrate. On the other hand, the solid electroless deposition method includes a solid substitution-type electroless plating method and a solid reduction-type electroless plating method. The solid substitution-type electroless plating method installs a porous film between a substitution-type electroless plating solution containing first metal ions and a second metal whose ionization tendency is larger than that of the first metal (or a second metal plated on a metal substrate). In this configuration, the first metal ions that have passed through the porous film cause a redox reaction, which is derived from a difference in ionization tendency between the metals, with the second metal as an underlying metal. By thus depositing the first metal ions on the surface of the second metal, a metal plating film made of the first metal is formed on the surface of the second metal. Alternatively, the solid reduction-type electroless plating method installs a porous film between a reduction-type electroless plating solution containing metal ions and a metal substrate. In this configuration, the metal ions that have passed through the porous film cause a redox reaction with a reductant contained in the reduction-type electroless plating solution. This method forms a metal plating film on the surface of the metal substrate by depositing the metal ions on the surface of the metal substrate.

The solid phase methods, such as the solid electrolyte deposition method and the solid electroless deposition method, use a small amount of a plating solution and a small amount of a waste liquid of the plating solution, and therefore a manufacturing cost and an environmental load can be reduced. For example, as in a film forming method of a metal plating film described in JP 2016-23338 A, in the conventional solid phase method, there may be a case where a film forming apparatus has a structure in which a porous film, such as a solid electrolyte membrane, is mounted so as to seal an opening end of a housing that houses a plating solution and the porous film is fitted by the housing.

SUMMARY

In the film forming apparatus having the structure described above, which is used in the conventional solid phase method, the porous film is possibly damaged due to the fitting by the housing. Additionally, due to its own weight of the plating solution, the porous film is possibly damaged. The damage of the porous film causes a leakage of the plating solution, failing to form the metal plating film. This problem becomes remarkable in a large-sized film forming apparatus.

The present disclosure has been made in view of the point, and the present disclosure provides a film forming method and a film forming apparatus of a metal plating film that allow suppressing damage of a porous film.

In order to solve the problem, a film forming method of a metal plating film according to the present disclosure is for forming a metal plating film on a surface of a metal substrate by solid substitution-type electroless plating method. The film forming method comprises preparing a film forming apparatus that includes a housing that includes at least a bottom wall and a sidewall surrounding the bottom wall and that is internally provided with a housing space, a metal substrate disposed on the bottom surface inside the housing, a porous film disposed on the surface of the metal substrate, and an electroless plating solution housed in the housing space; and using the film forming apparatus, reducing metal ions derived from the electroless plating solution contained in the porous film, and depositing the metal ions on the surface of the metal substrate to form the metal plating film on the surface of the metal substrate.

According to the film forming method of the metal plating film of the present disclosure, the damage of the porous film can be suppressed.

Furthermore, a film forming apparatus according to the present disclosure is for forming a metal plating film on a surface of a metal substrate by solid substitution-type electroless plating method. The film forming apparatus comprises a housing that includes at least a bottom wall and a sidewall surrounding the bottom wall and that is internally provided with a housing space, a metal substrate disposed on the bottom surface inside the housing, a porous film disposed on the surface of the metal substrate; and an electroless plating solution housed in the housing space.

According to the film forming apparatus of the present disclosure, the damage of the porous film can be suppressed.

Effect

According to the present disclosure, the damage of the porous film can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are schematic process cross-sectional views illustrating a film forming method of a metal plating film according to a first embodiment;

FIGS. 2A and 2B are schematic process cross-sectional views illustrating a film forming method of a metal plating film according to a second embodiment;

FIG. 3 is a schematic cross-sectional view illustrating a film forming apparatus according to a third embodiment;

FIGS. 4A to 4C are schematic process cross-sectional views illustrating a film forming method of a metal plating film according to Example 1;

FIG. 5 is a graph illustrating averages of weight changes between before and after film formation of 60 metal substrates in Example 1 (a housing made of PTFE is used and a porous film is present) and Comparative Example 1 (the housing made of PTFE is used and the porous film is absent), and Example 2 (a housing made of aluminum is used and the porous film is present) and Comparative Example 2 (the housing made of aluminum is used and the porous film is absent); and

FIG. 6 is a graph illustrating averages of surface roughnesses Ra of gold plating films of the 60 metal substrates in Example 1 (the housing made of PTFE is used and the porous film is present) and Comparative Example 1 (the housing made of PTFE is used and the porous film is absent), and Example 2 (the housing made of aluminum is used and the porous film is present) and Comparative Example 2 (the housing made of aluminum is used and the porous film is absent).

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments according to a film forming method and a film forming apparatus of a metal plating film according to the present disclosure will be described. First, outlines of the film forming method and the film forming apparatus of the metal plating film according to the embodiments will be described with a first embodiment and a second embodiment as examples.

First Embodiment

A film forming method of the metal plating film according to the first embodiment is a method for forming a metal plating film on a surface of a metal substrate by solid substitution-type electroless plating method and a film forming apparatus according to the first embodiment is a film forming apparatus configured to use the film forming method according to the first embodiment. FIGS. 1A to 1C are schematic process cross-sectional views illustrating the film forming method of the metal plating film according to the first embodiment.

The film forming method of the metal plating film according to the first embodiment first prepares a film forming apparatus 1 according to the first embodiment as illustrated in FIG. 1A (a preparing step). The film forming apparatus 1 includes a housing 2 that includes a bottom wall 2 bw and a sidewall 2 sw surrounding the bottom wall 2 bw and that is internally provided with a housing space 2S having a prism shape with a rectangular bottom surface 2 bs, a flat plate-shaped metal substrate 4 disposed on the bottom surface 2 bs inside the housing 2, a porous film 6 having a rectangular shape in plan view disposed on a surface 4 s of the metal substrate 4, a flat plate-shaped float member 8 having a rectangular shape in plan view disposed on a surface 6 s of the porous film 6, and an electroless gold plating solution L (an electroless plating solution) housed in the housing space 2S. The film forming apparatus 1 further includes a lid 10 that covers an opening 2 h opposed to the bottom wall 2 bw of the housing 2. The metal substrate 4, the porous film 6, and the float member 8 are disposed on the bottom surface 2 bs inside the housing 2 in the order in the vertical direction, housed in the housing space 2S of the housing 2, and immersed in the electroless gold plating solution L. In the metal substrate 4, a nickel plating film 4 n is formed on a surface of a copper substrate 4 c by electroless plating. The porous film 6 is not secured to any place but disposed on a surface 4 ns (the surface 4 s of the metal substrate 4) of the nickel plating film 4 n of the metal substrate 4. The electroless gold plating solution at least contains a gold compound and a complexing agent. The float member 8 has a density of 1.09 times or more and 1.65 times or less of a density of the electroless gold plating solution L, and the float member 8 has a weight larger than a weight of the porous film 6. The housing 2 is made of aluminum (a metal serving as a sacrificial anode). The lid 10 is made of the material same as that of the housing 2.

Next, as illustrated in FIG. 1B, the film forming apparatus 1 is loaded into an air atmosphere held at 80° C. in an inside 20N of a thermostatic oven 20. Accordingly, uniformly heating the housing 2 in the film forming apparatus 1 heats the electroless gold plating solution L and generates heat convection in the electroless gold plating solution L.

Next, as illustrated in FIG. 1C, using the film forming apparatus 1, while the heat convection is generated in the electroless gold plating solution L, gold ions (metal ions) derived from the electroless gold plating solution L contained in the porous film 6 are reduced to be deposited on the surface 4 ns (the surface 4 s of the metal substrate 4) of the nickel plating film 4 n of the metal substrate 4. This forms a gold plating film M (a metal plating film) on the surface 4 ns of the nickel plating film 4 n of the metal substrate 4 (a film forming step).

According to the first embodiment, the reduction and the deposition of the gold ions derived from the electroless gold plating solution L contained in the porous film 6, which is not secured to the housing 2, allow forming the gold plating film M. This allows suppressing damage of the porous film 6 used in the solid substitution-type electroless plating method.

According to the first embodiment, the film forming apparatus 1 further includes the float member 8. The density of the float member 8 is 1.09 times or more and 1.65 times or less of the density of the electroless gold plating solution L (the electroless plating solution) and the weight of the float member 8 is larger than the weight of the porous film 6. In view of this, when the gold ions are deposited on the surface 4 ns of the nickel plating film 4 n of the metal substrate 4 by reducing the gold ions derived from the electroless gold plating solution L contained in the porous film 6, while the float member 8 is brought into contact with a side surface 2 ss inside the housing 2 by heat convection of the electroless gold plating solution L, the float member 8 can be caused to perform a simple harmonic motion with an amplitude of 0.2 mm or less in the vertical direction. Specifically, the float member 8 is caused to perform the simple harmonic motion having the amplitude of 0.2 mm or less in the vertical direction at a position apart from the surface 6 s of the porous film 6 by a distance of the amplitude of 0.2 mm or less in the vertical direction as the center of the motion, and thus the float member 8 can repeatedly contact and separate from the surface 6 s of the porous film 6. Accordingly, in a state where the porous film 6 is not in tight contact with the surface 4 ns of the nickel plating film 4 n of the metal substrate 4 but approaches the surface 4 ns, the gold ions derived from the electroless gold plating solution L contained in the porous film 6 are reduced to allow depositing the gold ions on the surface 4 ns of the nickel plating film 4 n of the metal substrate 4. Therefore, in addition to allowing suppression of the damage of the porous film 6 in association with the tight contact with the metal substrate 4, the uniform gold plating film M can be formed.

Furthermore, according to the first embodiment, the housing 2 is made of aluminum (which is a metal serving as a sacrificial anode). In view of this, when the gold ions derived from the electroless gold plating solution L contained in the porous film 6 are reduced to be deposited on the surface 4 ns of the nickel plating film 4 n of the metal substrate 4, a back surface 4 cr (which is a back surface 4 r of the metal substrate 4) of the copper substrate 4 c where the nickel plating film 4 n is not disposed is in a state of being brought into contact with the bottom surface 2 bs of the housing 2 made of aluminum whose ionization tendency is larger than that of copper and nickel. This forms a local cell between the nickel plating film 4 n and the housing 2, and this local cell causes a local anode reaction of the housing 2. As a result of electrons generated by this reaction flowing from the housing 2 to the nickel plating film 4 n via the copper substrate 4 c, a local cathode reaction of the gold ions on the surface 4 ns of the nickel plating film 4 n is induced. In association with this, a substitution reaction between the gold and the nickel on the surface 4 ns of the nickel plating film 4 n is accelerated, thereby allowing film formation of the further uniform, thick gold plating film M. Additionally, the film forming apparatus 1 further includes the lid 10, which covers the opening 2 h in the housing 2, and the lid 10 is made of aluminum. This significantly accelerates the substitution reaction between the gold and the nickel and allows forming the much further uniform, thick gold plating film M.

Second Embodiment

A film forming method of a metal plating film according to the second embodiment is a method for forming a metal plating film on a surface of a metal substrate by solid substitution-type electroless plating method and a film forming apparatus according to the second embodiment is a film forming apparatus configured to use the film forming method according to the second embodiment. FIGS. 2A and 2B are schematic process cross-sectional views illustrating the film forming method of the metal plating film according to the second embodiment.

The film forming method of the metal plating film according to the second embodiment first prepares the film forming apparatus 1 according to the second embodiment as illustrated in FIG. 2A (the preparing step). The film forming apparatus 1 includes the housing 2 that includes the bottom wall 2 bw and the sidewall 2 sw surrounding the bottom wall 2 bw and that is internally provided with the housing space 2S having the prism shape with the rectangular bottom surface 2 bs, the flat plate-shaped metal substrate 4 disposed on the bottom surface 2 bs inside the housing 2, the porous film 6 having the rectangular shape in plan view disposed on the surface 4 s of the metal substrate 4, and the electroless gold plating solution L (the electroless plating solution) housed in the housing space 2S. The film forming apparatus 1 further includes the lid 10 that covers the opening 2 h opposed to the bottom wall 2 bw of the housing 2. The metal substrate 4 and the porous film 6 are disposed on the bottom surface 2 bs inside the housing 2 in the order in the vertical direction, housed in the housing space 2S of the housing 2, and immersed in the electroless gold plating solution L. In the metal substrate 4, the nickel plating film 4 n is formed on the surface of the copper substrate 4 c by electroless plating. The porous film 6 is not secured to any place but disposed on the surface 4 ns (the surface 4 s of the metal substrate 4) of the nickel plating film 4 n of the metal substrate 4. The electroless gold plating solution at least contains a gold compound and a complexing agent. A temperature of the electroless gold plating solution is set to 80° C. The housing 2 is made of polytetrafluoroethylene (PTFE). The lid 10 is made of the material same as that of the housing.

Next, as illustrated in FIG. 2B, using the film forming apparatus 1, the gold ions (the metal ions) derived from the electroless gold plating solution L contained in the porous film 6 are reduced to be deposited on the surface 4 ns (which is the surface 4 s of the metal substrate 4) of the nickel plating film 4 n of the metal substrate 4. This forms the gold plating film M (the metal plating film) on the surface 4 ns of the nickel plating film 4 n of the metal substrate 4 (the film forming step).

According to the second embodiment, the reduction and the deposition of the gold ions derived from the electroless gold plating solution L contained in the porous film 6, which is not secured to the housing 2, allow forming the gold plating film M. This allows suppressing damage of the porous film 6 used in the solid substitution-type electroless plating method.

Subsequently, details of the configurations of the film forming method of the metal plating film and the film forming apparatus according to the embodiment will be described.

1. Film Forming Method of Metal Plating Film

The film forming method of the metal plating film according to the embodiment is a film forming method of the metal plating film for forming the metal plating film on the surface of the metal substrate by solid substitution-type electroless plating method and includes the preparing step and the film forming step. Hereinafter, the preparing step and the film forming step will be described in detail.

(1) Preparing Step

In the preparing step, the film forming apparatus is prepared. The film forming apparatus includes a housing that includes at least the bottom wall and the sidewall surrounding the bottom wall and that is internally provided with the housing space, a metal substrate disposed on the bottom surface inside the housing, a porous film disposed on the surface of the metal substrate, and an electroless plating solution housed in the housing space.

a. Housing

Although a material constituting the housing is not specifically limited, and can be, for example, a metal or a resin, or the like. The housing is made of a metal serving as the sacrificial anode in some embodiments, because this allows forming a further uniform, thick metal plating film. As long as a metal whose ionization tendency is larger than that of the metal constituting the metal substrate is used, the metal serving as the sacrificial anode is not specifically limited. For example, in a case where the metal substrate includes a copper substrate with a surface on which a nickel plating film is disposed, the metal serving as the sacrificial anode is aluminum, iron, or the like in some embodiments.

Note that since an electroless plating solution described later is housed in the housing space inside the housing, oxidation of the plating solution can be suppressed. Therefore, an oxidation inhibitor need not to be added to the electroless plating solution. Moreover, sealing the electroless plating solution with the housing allows facilitating generation of hydrogen co-deposition in the plating film, and as a result, solder wettability can be improved.

The film forming apparatus that further includes the lid covering the opening of the housing is used in some embodiments. Furthermore, in a case where the housing is made of the metal serving as the sacrificial anode, the lid is made of the material same as that of the housing in some embodiments. This is because the lid functions as the sacrificial anode together with the housing, and therefore the further uniform, thick gold plating film can be formed.

The shape of the housing space in the housing is not specifically limited, and examples of which include a prism with a rectangular bottom surface and a column with a circular bottom surface. The size of the bottom surface of the housing space is not specifically limited, and, for example, with the bottom surface having the rectangular shape, the longitudinal size is 1 cm or more and 100 cm or less and the lateral size is 1 cm or more and 100 cm or less in some embodiments. With the bottom surface having the circular shape, the diameter is 1 cm or more and 100 cm or less in some embodiments. This is because the size of the bottom surface of the housing space within the range facilitate film formation of the uniform metal plating film when the film forming apparatus includes the float member. A depth of the housing space can be set to a depth required to house the electroless plating solution at a depth required for film formation of the desired metal plating films.

Here, FIG. 3 is a schematic cross-sectional view illustrating a film forming apparatus according to a third embodiment. As illustrated in FIG. 3, the film forming apparatus further includes a seal 12 disposed inside the housing 2 in some embodiments. As illustrated in FIG. 3, the seal 12 is internally in contact with the side surface 2 ss inside the housing 2 and externally in contact with a surface, such as an outer peripheral surface 4 p of the metal substrate 4, in some embodiments. An amount of dissolution of the metal ions of the constituent material (which is the metal serving as the sacrificial anode of aluminum or the like) of the housing 2 to the electroless gold plating solution L (which is a substitution-type electroless gold plating bath) interposed between the side surface 2 ss of the housing 2 and the outer peripheral surface 4 p of the metal substrate 4 is larger than an amount of dissolution of the metal ions of the constituent material of the housing 2 to the electroless gold plating solution L on an upper side of a surface 8 s of the float member 8. In view of this, a deterioration rate of the electroless gold plating solution L interposed between the side surface 2 ss of the housing 2 and the outer peripheral surface 4 p of the metal substrate 4 is faster than that of the electroless gold plating solution L on the upper side of the surface 8 s of the float member 8. When the metal ions of the constituent material of the housing 2 are dissolved into a liquid L2, such as ion exchanged water (an unsubstitution type electroless gold plating bath) filled in a region surrounded by the side surface 2 ss inside the housing 2, the seal 12, and the outer peripheral surface 4 p of the metal substrate 4 instead of the electroless gold plating solution L, the gold plating film M can be formed on the metal substrate 4. In view of this, as illustrated in FIG. 3, the film forming apparatus further includes the seal 12, and the liquid L2, such as the ion exchanged water, is filled in the region surrounded by the side surface 2 ss inside the housing 2, the seal 12, and the outer peripheral surface 4 p of the metal substrate 4 in some embodiments. This allows reducing the use amount of the electroless gold plating solution L.

The constituent material of the seal 12 is not specifically limited, and is, for example, an elastomer, a flexible polymer, and a polymer foam in some embodiments.

b. Electroless Plating Solution

The electroless plating solution is a plating solution used in the substitution-type electroless plating method. For example, the electroless plating solution contains a metal compound and a complexing agent and may contain an additive as necessary. Examples of the additive include a pH buffer and a stabilizer. The commercially available plating solution may be used. The electroless plating solution is, for example, an electroless gold plating solution. Hereinafter, the electroless gold plating solution will be described in detail.

The electroless gold plating solution at least contains a metal compound and a complexing agent and may contain an additive as necessary.

While the gold compound is not specifically limited, the gold compound includes, for example, a cyanide gold salt or a non-cyanide gold salt. The cyanide gold salt includes gold cyanide, gold potassium cyanide, gold sodium cyanide, ammonium gold cyanide, or the like. The non-cyanide gold salt includes, for example, a gold sulfite salt, a gold thiosulfate salt, a chloroaurate, a gold thiomalate, or the like. One kind of gold salt may be used alone, or two or more kinds thereof may be used in combination. As the gold salt, from the aspects of handling, environment, and toxicity, the non-cyanide gold salt is used in some embodiments, and among the non-cyanide gold salts, the gold sulfite salt is used in some embodiments. The gold sulfite salt can include, for example, ammonium gold sulfite, potassium gold sulfite, gold sodium sulfite, methanesulfonic acid gold salt, or the like.

The content of the gold compound in the electroless gold plating solution is 0.5 g/L or more and 2.5 g/L or less as gold in some embodiments, and 1.0 g/L or more and 2.0 g/L or less in some embodiments. The respective upper limit values and lower limit values of these numerical ranges can be freely combined to specify an appropriate range. When the gold content is 0.5 g/L or more, a gold deposition reaction can be improved. When the gold content is 2.5 g/L or less, stability of the plating solution can be improved.

The complexing agent provides an effect to stably complex gold ions (Au⁺) to decrease the occurrence of a disproportionation reaction of Au⁺ (3Au⁺→Au³⁺+2Au), thereby improving the stability of the liquid. One kind of the complexing agent may be used alone, or two or more kinds thereof may be used in combination.

The complexing agent includes, for example, a cyanide complexing agent or a non-cyanide complexing agent. The cyanide complexing agent includes, for example, sodium cyanide or potassium cyanide. The non-cyanide complexing agent includes, for example, sulfite, thiosulfate, thiomalate, thiocyanate, mercaptosuccinic acid, mercaptoacetic acid, 2-mercaptopropionic acid, 2-aminoethanethiol, 2-mercaptoethanol, glucose cysteine, 1-thioglycerol, sodium mercaptopropane sulfonate, N-acetyl methionine, thiosalicylic acid, ethylenediaminetetraacetic acid (EDTA), or pyrophosphoric acid. As the complexing agent, from the aspect of handling, environment, and toxicity, the non-cyanide complexing agent is used in some embodiments, and the sulfite among the non-cyanide complexing agent is used in some embodiments.

The content of the complexing agent in the electroless gold plating solution is 1 g/L or more and 200 g/L or less in some embodiments, and is 20 g/L or more and 50 g/L or less in some embodiments. The respective upper limit values and lower limit values of these numerical ranges can be freely combined to specify an appropriate range. When the content of the complexing agent is 1 g/L or more, a gold complexing ability is increased to allow improving the stability of the plating solution. When the content of the complexing agent is 200 g/L or less, generation of recrystallization in the plating solution can be suppressed.

The electroless gold plating solution can contain an additive as necessary. The additive includes, for example, a pH buffer or a stabilizer.

The pH buffer can adjust a deposition rate to a desired value, and can keep pH of the plating solution constant. One kind of the pH buffer may be used alone, or two or more kinds thereof may be used in combination. The pH buffer includes, for example, phosphate, acetate, carbonate, borate, citrate, or hydrosulfate.

The pH of the electroless gold plating solution is 5.0 or more and 8.0 or less in some embodiments, 6.0 or more and 7.8 or less in some embodiments, or 6.8 or more and 7.5 or less in some embodiments. The respective upper limit values and lower limit values of these numerical ranges can be freely combined to specify an appropriate range. When the pH is 5.0 or more, the stability of the plating solution tends to be improved. When the pH is 8.0 or less, corrosion of the metal substrate as the underlying metal can be suppressed. The pH can be adjusted by adding, for example, potassium hydroxide, sodium hydroxide, and ammonium hydroxide.

The stabilizer can improve the stability of the plating solution. The stabilizer includes, for example, a thiazole compound, a bipyridyl compound, or a phenanthroline compound.

A commercially available electroless gold plating solution may be used. The commercial product includes, for example, EPITHAS TDS-25 (manufactured by C. Uyemura & Co., Ltd.), EPITHAS TDS-20 (manufactured by C. Uyemura & Co., Ltd.), or FLASH GOLD (manufactured by OKUNO CHEMICAL INDUSTRIES CO., LTD.).

A depth of the electroless plating solution housed in the housing space in the housing is not specifically limited, but is 0.5 cm or more and 100 cm or less in some embodiments. It is because the depth of the electroless plating solution within the range facilitates forming the uniform metal plating film when the film forming apparatus includes the float member.

c. Metal Substrate

The metal substrate is made of a metal whose ionization tendency is larger than that of the metal constituting the metal plating film such that the metal plating film can be formed on the surface of the metal substrate by solid substitution-type electroless plating method. As long as the metal constituting the metal substrate is the above-described metal, the metal constituting the metal substrate is not specifically limited. For example, when the metal constituting the metal plating film is gold, examples of the metal constituting the metal substrate include copper, nickel, cobalt, palladium, and an alloy containing at least two kinds of them. Among them, nickel, a nickel alloy, or the like is used in some embodiments. It is because the nickel or the nickel alloy allows facilitating the formation of the gold plating film. As long as the metal substrate is the above-described metal substrate, the metal substrate is not specifically limited. For example, when the metal constituting the metal plating film is gold, examples of the metal substrate includes a metal substrate in which a nickel plating film is disposed on the surface of the copper substrate.

The metal substrate can have any shape. Examples of the shape of the metal substrate include a plate shape, such as a flat plate shape or a curved plate shape, a rod shape, or a spherical shape. The metal substrate may be an object on which fine processing, such as a groove and a hole, is performed, and may be, for example, a wiring for an electronic industry component, such as a printed wiring board, an ITO substrate, and a ceramic IC package substrate. The metal substrate may be a plating film formed on a resin product, a glass product, or a product such as a ceramic component.

d. Porous Film

As long as the porous film that can internally contain the electroless plating solution and allows the metal ions to be deposited on the surface of the metal substrate by reducing the metal ions derived from the electroless plating solution contained in the porous film, the porous film is not specifically limited, but the porous film having an anionic group is used in some embodiments. The porous film having the anionic group allows the anionic group to capture the metal ions dissolved from the metal substrate. This allows suppressing deterioration of the electroless plating solution by the metal ions (for example, nickel ions) derived from the metal substrate. Moreover, since the porous film having the anionic group is hydrophilic, wettability of the porous film is improved. In view of this, with the porous film having the anionic group, the electroless plating solution easily wets, and the electroless plating solution can be uniformly spread on the surface of the metal substrate. As a result, the porous film having the anionic group also provides an effect of allowing forming the uniform metal plating film.

While the anionic group is not specifically limited, the anionic group is at least one kind selected from, for example, sulfonate group, thiosulfonic group (—S₂O₃H), carboxy group, phosphate group, phosphonate group, hydroxy group, cyano group, and thiocyano group. These anionic groups can capture metal ions having positive electric charges. These anionic groups can give the hydrophilicity to the porous film. The anionic group comprise sulfonate group or carboxy group in some embodiments. Especially, the anionic group comprise sulfonate group (sulfo group) in some embodiments because nickel ions can be effectively captured.

As a material of the porous film having the anionic group, an anionic polymer can be used. That is, the porous film having the anionic group includes the anionic polymer. The anionic polymer has the anionic group (for example, the sulfonate group, the thiosulfonic group, the carboxy group, the phosphate group, the phosphonate group, the hydroxy group, the cyano group, or the thiocyano group, described above). The anionic polymer may have one kind of the anionic groups alone, or may have two kinds or more of the anionic groups in combination. The anionic group is the sulfonate group in some embodiments.

While the anionic polymer is not specifically limited, the anionic polymer can includes, for example, a polymer containing a monomer having the anionic group.

Representatively, examples of the anionic polymer include, for example, a polymer having carboxyl group [for example, a (meth)acrylic acid polymer (for example, a copolymer of (meth)acrylic acid and another copolymerizable monomer such as poly(meth)acrylic acid), or a fluorine-based resin having carboxyl group (for example, perfluorocarboxylic acid resin)], a styrene-based resin having sulfonate group [for example, polystyrene sulfonic acid], and a sulfonated polyarene ether resin [for example, sulfonated polyether ketone resin, sulfonated polyethersulfone resin].

Among the porous films having the anionic groups, a solid electrolyte membrane having ionic conductivity is used in some embodiments. The solid electrolyte membrane internally has an ion cluster structure, and the plating solution is impregnated to the inside of the ion cluster structure. Since the metal ions, such as gold ions, in the plating solution are coordinated to the anionic group in the solid electrolyte membrane, the metal ions are effectively spread in the solid electrolyte membrane. Therefore, the use of the solid electrolyte membrane allows the formation of the uniform plating film.

The solid electrolyte membrane has a porous structure (that is, an ion cluster structure), and pores of the porous structure are very small, having an average pore diameter of, for example, 0.1 μm or more and 100 μm or less. By applying a pressure, the electroless plating solution can be impregnated into the solid electrolyte membrane. While the solid electrolyte membrane can include, for example, a fluorine-based resin, such as Nafion (registered trademark) manufactured by DuPont de Nemours, Inc., a hydrocarbon resin, a polyamic acid resin, and a resin having an ion exchange function, such as Selemion (CMV, CMD, CMF series) manufactured by AGC Inc., the solid electrolyte membrane is not specifically limited to them. The solid electrolyte membrane is the fluorine-based resin having the sulfonate group in some embodiments. The fluorine-based resin having the sulfonate group has a hydrophobic part of a fluorinated carbon skeleton and a hydrophilic part of a side chain part having the sulfonate group, and these parts form the ion cluster. The metal ions in the plating solution impregnated to the inside of the ion cluster are coordinated to the sulfonate group of the solid electrolyte membrane, and uniformly spread in the solid electrolyte membrane. Since the solid electrolyte membrane having the sulfonate group is easily wettable by the plating solution because of high hydrophilicity and excellent wettability, the plating solution can be uniformly spread on the surface of the metal substrate. Therefore, the use of the fluorine-based resin having the sulfonate group allows the formation of the uniform plating film. The use of the fluorine-based resin having the sulfonate group increases dielectric polarization generated at a diffusion layer present between the solid electrolyte membrane and the metal substrate due to Maxwell-Wagner effect, thus allowing high speed transport of the metal ions. Such a fluorine-based resin is available as, for example, a series of a product name “Nafion” from DuPont de Nemours, Inc.

The equivalent weight (EW) of the solid electrolyte membrane is 850 g/mol or more and 950 g/mol or less in some embodiments, and is 874 g/mol or more and 909 g/mol or less in some embodiments. The respective upper limit values and lower limit values of these numerical ranges can be freely combined to specify an appropriate range. Here, the equivalent weight means a dry mass of the solid electrolyte membrane per equivalent of an ion exchange group. When the equivalent weight of the solid electrolyte membrane is in this range, the uniformity of the metal plating film can be improved.

While an adjustment method of the equivalent weight of the solid electrolyte membrane is not specifically limited, for example, in the case of a perfluorocarbon sulfonic acid polymer, the adjustment can be performed by changing a polymerization ratio between a fluorinated vinyl ether compound and a fluorinated olefin monomer. Specifically, for example, by increasing the polymerization ratio of the fluorinated vinyl ether compound, the equivalent weight of the solid electrolyte membrane to be obtained can be decreased. The equivalent weight can be measured using a titration method.

The density of the porous film is not specifically limited, but is 0.005 times or more and 0.9 times or less of the density of the electroless plating solution in some embodiments, and in particular, 0.01 times or more and 0.8 times or less of the density of the electroless plating solution in some embodiments. It is because this effectively achieves bringing the porous film close to the surface of the metal substrate by the float member without tight contact.

The porous film has a thickness of 10 μm or more and 200 μm or less in some embodiments and 20 μm or more and 160 μm or less in some embodiments. The respective upper limit values and lower limit values of these numerical ranges can be freely combined to specify an appropriate range. When the thickness of the porous film is 10 μm or more, the porous film is less likely to be torn and is excellent in durability. The thickness of the porous film of 200 μm or less allows reducing a pressure required for the electroless plating solution to pass through the porous film.

While the shape and the size of the porous film in plan view are not specifically limited, for example, when the shape of the housing space in the housing is the prism with the rectangular bottom surface, the shape of the porous film in plan view is a rectangular shape, the longitudinal size of the porous film in plan view is (the longitudinal size of the bottom surface of the housing space—10 mm) or more and equal to or less than the longitudinal size of the bottom surface of the housing space, and the lateral size of the porous film in plan view is (the lateral size of the bottom surface of the housing space—10 mm) or more and equal to or less than the lateral size of the bottom surface of the housing space in some embodiments. It is because the vertical movement of the float member while the float member is brought into contact with the side surface inside the housing facilitates the formation of the uniform metal plating film. An example of the size of the porous film includes a size of longitudinal 190 mm×lateral 280 mm×thickness 51 μm. As the shape and the size of the porous film in plan view, for example, when the shape of the housing space in the housing is a column with a circular bottom surface, the shape of the porous film in plan view is a circular shape, a size of a diameter of the porous film in plan view is (the diameter of the bottom surface of the housing space—10 mm) or more and equal to or less than the diameter of the bottom surface of the housing space in some embodiments. It is because the vertical movement of the float member while the float member is brought into contact with the side surface inside the housing facilitates the formation of the uniform metal plating film.

A water contact angle of the porous film is 15° or less in some embodiments, 13° or less in some embodiments, and 10° C. or less in some embodiments. The water contact angle of the porous film within the range allows improving the wettability of the porous film.

e. Float Member

The film forming apparatus further includes the float member disposed on the surface of the porous film, the density of the float member is 1.09 times or more and 1.65 times or less of the density of the electroless plating solution, and the weight of the float member is larger than the weight of the porous film in some embodiments. In the case where the float member is further provided, when the metal ions derived from the electroless plating solution contained in the porous film are reduced to be deposited on the surface of the metal substrate, in association with the movement of the float member by heat convection of the electroless plating solution, the porous film approaches the surface of the metal substrate without tight contact of the porous film with the surface of the metal substrate. Accordingly, by forming a sufficient diffusion layer between the metal substrate and the porous film, a gas, such as hydrogen, generated in the deposition of the metal ions can be released from between the metal substrate and the porous film, and therefore the uniform metal plating film can be formed. Here, “tight contact of the porous film with the surface of the metal substrate” refers to a case in which, assuming that a total interaction energy (a sum of an electric double layer repulsive force and van der Waals attraction) between the surfaces of the porous film and the nickel plating film in the electroless gold plating solution is a function having the inter-surface distance as an argument, the inter-surface distance becomes equal to or less than a distance (usually, the distance of 1 nm or more and 4 nm or less) at which the total interaction energy becomes the local maximum value (an energy barrier) (Derjaguin, B. V. and Landau, L. (1941). Acta Physicochim. URSS14, 633-662). An electrostatic double layer repulsive force between two charged surfaces separated by a solvent containing a counter ion, such as an electroless gold plating bath, is given by a contact value theorem. When the two surfaces approach one another to change the electroless gold plating solution density on the surfaces, a solvation pressure is generated (Evans, R. and Parry, A. O. (1990). J. Phys.: Condens. Matter. 2, SA15-SA32.). The solvation pressure is an oscillating function having the electroless gold plating solution density on the surfaces as an argument, in which the inter-surface distance spans a distance multiples of a molecular diameter. In a case where the inter-surface distance is considerably small, the solvation pressure becomes to have a negative finite value, becoming an attachment force. Therefore, when the inter-surface distance between the porous film and the metal substrate is reduced, the solvation pressure becomes oscillational. Since the float member performs a natural circular oscillation by heat convection, as long as the solvation pressure synchronizes with the own natural period of the float member, the attachment force of the contact surfaces can be constant.

In a case where the density of the float member is less than the lower limit of the range, when the metal ions derived from the electroless plating solution contained in the porous film are reduced to be deposited on the surface of the metal substrate, the float member floats from the surface of the porous film by the heat convection of the electroless plating solution and this possibly generates a part where the metal plating film is not formed. In a case where the density of the float member exceeds the upper limit of the range, when the metal ions derived from the electroless plating solution contained in the porous film are reduced to be deposited on the surface of the metal substrate, despite the heat convection of the electroless plating solution, the float member is in tight contact with the porous film. This brings the porous film into tight contact with the surface of the metal substrate and therefore the sufficient diffusion layer is not formed between the metal substrate and the porous film, possibly failing to form the uniform metal plating film. Additionally, in a case where the weight of the float member is equal to or less than the weight of the porous film, when the metal ions derived from the electroless plating solution contained in the porous film are deposited on the surface of the metal substrate, bringing the porous film close to the surface of the metal substrate by the float member becomes difficult, possibly failing to form the uniform metal plating film.

As long as the constituent material of the float member has a chemical resistance against the electroless plating solution, the constituent material is not specifically limited, and may be an organic material or an inorganic material, but an organic material, such as a resin, is used in some embodiments. It is because such an organic material has a high chemical resistance. The organic material of the float member is not specifically limited and different depending on the kind of the electroless plating solution. Examples of the organic material of the float member include PA66 (nylon 66), a phenolic resin, polyethylene terephthalate (PET), or polyvinylidene chloride (PVDC) when the electroless plating solution is EPITHAS TDS-25 manufactured by C. Uyemura & Co., Ltd. or the like.

As long as the uniform metal plating film can be formed, the shape of the float member is not specifically limited, and is a plate shape, such as a flat plate shape or a curved plate shape. The float member that does not have a hole, such as a through-hole, which possibly makes the movement of the float member irregular when the float member moves due to the heat convection of the electroless plating solution and possibly blocks the formation of the desired, uniform metal plating film, is used in some embodiments. The hole has a pore diameter of, for example, 1 μm or more.

The thickness of the float member is 0.5 mm or more and 10 mm or less in some embodiments and 1.0 mm or more and 5.0 mm or less in some embodiments. The respective upper limit values and lower limit values of these numerical ranges can be freely combined to specify an appropriate range. It is because, when the thickness of the float member is the lower limit value or more of these numerical ranges, it can be suppressed that the float member is excessively light and this makes it difficult to bring the porous film close to the surface of the metal substrate by the float member. It is because, when the thickness of the float member is the upper limit value or less of these numerical ranges, it can be suppressed that the float member is excessively heavy and the porous film and the metal substrate are in tight contact. Therefore, the thickness of the float member within the numerical ranges sinks the float member into the liquid, and the float member can vibrate without causing the tight contact of the porous film with the metal substrate. The shape of the float member is a flat plate shape in some embodiments, rather than a curved plate shape. It is because, in the plating solution, a force that acts on the surface of the flat plate-shaped float member is equal to a product of a pressure at the center of gravity of the surface of the float member and an area of the surface of the float member and orthogonally acts on the surface of the float member. Accordingly, the uniform metal plating film is easily formed.

While the shape and the size of the float member in plan view are not specifically limited, when the float member and the porous film are in plan view, the shape and the size at which the region of the porous film falls within the region of the float member are used in some embodiments. As the shape and the size of the float member in plan view, for example, when the shape of the housing space in the housing is the prism with the rectangular bottom surface, the shape of the float member in plan view is a rectangular shape, the longitudinal size of the float member in plan view is (the longitudinal size of the bottom surface of the housing space—10 mm) or more and equal to or less than the longitudinal size of the bottom surface of the housing space, and the lateral size of the float member in plan view is (the lateral size of the bottom surface of the housing space—10 mm) or more and equal to or less than the lateral size of the bottom surface of the housing space in some embodiments. It is because the vertical movement of the float member while the float member is brought into contact with the side surface inside the housing facilitates the formation of the uniform metal plating film. An example of the size of the float member includes a size of longitudinal 195 mm×lateral 282 mm×thickness 2 mm. As the shape and the size of the float member in plan view, for example, when the shape of the housing space in the housing is a column with a circular bottom surface, the shape of the float member in plan view is a circular shape, a size of a diameter of the float member in plan view is (the diameter of the bottom surface of the housing space—10 mm) or more and equal to or less than the diameter of the bottom surface of the housing space in some embodiments. It is because the vertical movement of the float member while the float member is brought into contact with the side surface inside the housing facilitates the formation of the uniform metal plating film.

f. Others

The film forming apparatus in which the electroless gold plating solution contains gold sulfite salt as a gold compound and/or sulfite salt as a complexing agent, and the porous film is a solid electrolyte membrane having the sulfonate group as an anionic group is used in some embodiments. The gold sulfite salt and the sulfite salt are easily impregnated into the solid electrolyte membrane having the sulfonate group, and further, the gold ion is coordinated to the sulfonate group to be effectively spread in the solid electrolyte membrane. Therefore, the gold ion is sufficiently supplied to the film formation part to allow uniformly forming the plating film.

The film forming apparatus in which the electroless gold plating solution contains a carboxyl group-containing compound as a complexing agent, and the porous film is a solid electrolyte membrane having carboxy group as an anionic group is used in some embodiments. The carboxyl group-containing compound includes, for example, mercaptosuccinic acid, acetylcysteine, or cysteine in addition to the compounds listed above. The carboxyl group-containing compound can form a stable complex with gold ions. By combining the solid electrolyte membrane having carboxyl group and the gold plating solution containing the carboxyl group-containing compound, the plating solution can be stably kept at a mild acidity, thereby allowing the formation of the uniform plating film. The carboxyl group-containing compound is easily impregnated into the solid electrolyte membrane having carboxyl group, and further, the gold ion is coordinated to the carboxyl group to be effectively spread in the solid electrolyte membrane. Therefore, the gold ion is sufficiently supplied to the film formation part to allow uniformly forming the plating film.

(2) Film Forming Step

In the film forming step, using the film forming apparatus, the metal ions derived from the electroless plating solution contained in the porous film are reduced to be deposited on the surface of the metal substrate, thus forming the metal plating film on the surface of the metal substrate.

A plating temperature (an electroless plating temperature when the metal plating film is formed) is, for example, 50° C. or more and 95° C. or less, and 60° C. or more and 90° C. or less in some embodiments. The respective upper limit values and lower limit values of these numerical ranges can be freely combined to specify an appropriate range. The plating temperature of 50° C. or more allows improving the deposition rate of the metal plating film. The plating temperature of 95° C. or less allows suppressing decomposition of the component in the plating solution. While a plating time depends on the plating temperature, an example of which is from 1 to 60 minutes.

In the film forming step, by loading the film forming apparatus inside the thermostatic oven, while the heat convection is generated in the electroless plating solution, the metal ions derived from the electroless plating solution contained in the porous film are reduced to be deposited on the surface of the metal substrate in some embodiments. It is because this allows suppressing strong heat convection when the electroless plating solution is heated and achieving the formation of the uniform metal plating film. As the thermostatic oven, for example, an thermostatic oven inside of which is under the air atmosphere with the temperature held at the above-described plating temperature is used in some embodiments.

2. Film Forming Apparatus

The film forming apparatus according to the embodiment further includes the float member disposed on the surface of the porous film, the density of the float member is 1.09 times or more and 1.65 times or less of the density of the electroless plating solution, and the weight of the float member is larger than the weight of the porous film in some embodiments. The film forming apparatus in which the housing is made of the metal serving as the sacrificial anode is used in some embodiments. Among the film forming apparatuss, the film forming apparatus that further includes the lid that covers the opening of the housing and the lid is made of the material same as the housing is used in some embodiments. Additionally, the lid of the housing may be connected to the center of gravity of the float member with a spring, a natural circular frequency when the float member is suspended in the electroless gold plating solution may be set smaller than a natural circular frequency in pure water, and the float member may be caused to perform natural circular oscillation in an electroless gold plating bath.

EXAMPLES

The following will describe this embodiment with examples, but the present disclosure is not limited to the examples.

Example 1

A film forming method that formed gold plating films (metal plating films) on surfaces of 60 metal substrates by solid substitution-type electroless plating method was performed. FIGS. 4A to 4C are schematic process cross-sectional views illustrating the film forming method of the metal plating film according to Example 1.

This film forming method of the gold plating film first prepared the film forming apparatus 1 as illustrated in FIG. 4A (the preparing step). The film forming apparatus 1 includes the housing 2. The housing 2 includes the bottom wall 2 bw, the sidewall 2 sw that surrounds the bottom wall 2 bw, and the opening 2 h opposed to the bottom wall 2 bw, and internally provides the prism-shaped housing space 2S with the rectangular bottom surface 2 bs. The film forming apparatus 1 includes the 60 flat plate-shaped metal substrates 4 disposed on the bottom surface 2 bs inside the housing 2, the porous film 6 having the rectangular shape in plan view disposed on the surfaces 4 s of the 60 metal substrates 4, the flat plate-shaped float member 8 having the rectangular shape in plan view disposed on the surface 6 s of the porous film 6, and the electroless gold plating solution (the electroless plating solution) L housed in the housing space 2S in the housing 2. The film forming apparatus 1 further includes the lid 10 that covers the opening 2 h. The metal substrates 4, the porous film 6, and the float member 8 are disposed on the bottom surface 2 bs inside the housing 2 in the order in the vertical direction, housed in the housing space 2S of the housing 2, and immersed in the electroless gold plating solution L. In the metal substrate 4, the nickel plating film 4 n is formed on a surface of the copper block substrate 4 c by electroless plating. The porous film 6 is not secured to any place but disposed on the surfaces 4 ns (the surfaces 4 s of the metal substrates 4) of the nickel plating films 4 n of the 60 metal substrates 4. The electroless gold plating solution at least contains a gold compound and a complexing agent. Note that the 60 metal substrates 4 are disposed in 4 rows×15 columns on the bottom surface 2 bs inside the housing 2 in plan view, and FIGS. 4A to 4C illustrate the 15 metal substrates 4 disposed in one row. The following will describe the configurations of the respective members and the electroless gold plating solution used in the film forming apparatus 1.

(Housing) Constituent Material: Polytetrafluoroethylene (PTFE)

Size of housing space: longitudinal 200 mm×lateral 284 mm×depth 9.2 mm

(Metal Substrate)

Configuration: nickel plating copper block on which a nickel plating film is formed on the surface of the copper block substrate by electroless plating Size of copper block substrate: longitudinal 18 mm×lateral 34.5 mm×thickness 3 mm Thickness of the nickel plating film: 80 nm Film formation conditions of nickel plating film:

-   -   Nickel plating solution: Top Nicoron TOM-LF (manufactured by         OKUNO CHEMICAL INDUSTRIES CO., LTD.)     -   Plating temperature: 90° C.     -   Film formation time: 15 minutes

(Porous Film)

Constituent material: Nafion (registered trademark) NRE-212 (manufactured by DuPont de Nemours, Inc.) Size: longitudinal 190 mm×lateral 280 mm×thickness 51 μm [the size equal to or less than the size of the housing space] Density: 0.01 g/cm³ [the density smaller than that of the float member] Weight: 5.3 g [the weight smaller than that of the float member] (Float member) Constituent material: PA66 (nylon 66) Size: longitudinal 195 mm×lateral 282 mm×thickness 2 mm [the size equal to or less than the size of the housing space] Density: 1.13 g/cm³ [the density larger than that of the porous film] Weight: 124 g [the weight larger than that of the porous film]

Through-Hole: Absent (Electroless Gold Plating Solution)

Type: EPITHAS TDS-25 (manufactured by C. Uyemura & Co., Ltd.) Density: 1.04 g/cm³ Depth: 9.2 mm [The inside of the housing was filled with an electroless gold plating bath.]

-   -   EPITHAS TDS-25 contains TDS-25-M (a mixture of oxalate,         alkylaminophosphonic acid, alkylaminophosphonic acid salt, and         water), TDS-25-A (a mixture of sulfite and water), and a gold         sodium sulfite solution.

Next, as illustrated in FIG. 4B, the film forming method of the gold plating film loaded the film forming apparatus 1 in the inside 20N in the thermostatic oven 20 under the air atmosphere held at 80° C. By thus uniformly heating the housing 2 in the film forming apparatus 1, the electroless gold plating solution L was heated to generate the heat convection in the electroless gold plating solution L.

Next, as illustrated in FIG. 4C, using the film forming apparatus 1, while the heat convection was generated in the electroless gold plating solution L, the gold ions (the metal ions) derived from the electroless gold plating solution L contained in the porous film 6 were reduced to be deposited on the surfaces 4 ns of the nickel plating films 4 n of the 60 metal substrates 4 (the surfaces 4 s of the metal substrates 4). Thus, the gold plating films M (the metal plating films) were formed on the surfaces 4 ns of the nickel plating films 4 n of the 60 metal substrates 4 (the film forming step). In this respect, the film formation time was set to 15 minutes, and the film formation area of each metal substrate 4 was set to the longitudinal 18 mm×lateral 34.5 mm. Consequently, the gold plating films M were able to be formed without causing damage of the porous film 6.

Example 2

Except that a housing in which a constituent material was changed as follows was used as the housing, the film forming method that formed the gold plating films on the surfaces of the 60 metal substrates was performed similarly to Example 1. Consequently, the gold plating films were able to be formed without causing damage of the porous film.

(Housing)

Constituent material: aluminum (A1050)

Example 3

Except that a float member in which a constituent material, a density, and a weight were changed as follows was used as the float member, the film forming method that formed the gold plating films on the surfaces of the 60 metal substrates was performed similarly to Example 2. Consequently, the gold plating films were able to be formed without causing damage of the porous film.

(Float Member)

Constituent material: Polytetrafluoroethylene (PTFE) Density: 2.14 g/cm³ [the density larger than that of the porous film] Weight: 235 g [the weight larger than that of the porous film]

Example 4

Except that a float member in which a constituent material, a density, and a weight were changed as follows was used as the float member, the film forming method that formed the gold plating films on the surfaces of the 60 metal substrates was performed similarly to Example 2. Consequently, the gold plating films were able to be formed without causing damage of the porous film.

(Float Member)

Constituent material: polypropylene (PP) Density: 0.90 g/cm³ [the density larger than that of the porous film] Weight: 99.0 g [the weight larger than that of the porous film]

Example 5

Except that a float member in which a constituent material, a density, and a weight were changed as follows was used as the float member, the film forming method that formed the gold plating films on the surfaces of the 60 metal substrates was performed similarly to Example 2. Consequently, the gold plating films were able to be formed without causing damage of the porous film.

(Float Member)

Constituent material: phenolic resin Density: 1.21 g/cm³ [the density larger than that of the porous film] Weight: 133 g [the weight larger than that of the porous film]

Example 6

Except that a float member in which a constituent material, a density, and a weight were changed as follows was used as the float member, the film forming method that formed the gold plating films on the surfaces of the 60 metal substrates was performed similarly to Example 2. Consequently, the gold plating films were able to be formed without causing damage of the porous film.

(Float Member)

Constituent material: polyethylene terephthalate (PET) Density: 1.34 g/cm³ [the density larger than that of the porous film] Weight: 147 g [the weight larger than that of the porous film]

Example 7

Except that a float member in which a constituent material, a density, and a weight were changed as follows was used as the float member, the film forming method that formed the gold plating films on the surfaces of the 60 metal substrates was performed similarly to Example 2. Consequently, the gold plating films were able to be formed without causing damage of the porous film.

(Float Member)

Constituent material: polyvinylidene chloride (PVDC) Density: 1.72 g/cm³ [the density larger than that of the porous film] Weight: 189 g [the weight larger than that of the porous film]

Comparative Example 1

First, except that the porous film 6 was not provided, the float member 8 was disposed on the surfaces 4 s of the 60 metal substrates 4 without via the porous film 6, and the metal substrates 4 and the float member 8 were disposed on the bottom surface 2 bs inside the housing 2 in the order in the vertical directions, housed in the housing space 2S in the housing 2, and immersed into the electroless gold plating solutions L, the film forming apparatus 1 similar to that of Example 1 was prepared (the preparing step).

Next, the film forming apparatus 1 was loaded in the inside 20N in the thermostatic oven 20 under the air atmosphere held at 80° C. By thus uniformly heating the housing 2 in the film forming apparatus 1, the electroless gold plating solution L was heated to generate the heat convection in the electroless gold plating solution L.

Next, using the film forming apparatus 1, while the heat convection was generated in the electroless gold plating solution L, the gold ions (the metal ions) derived from the electroless gold plating solution L supplied without via the porous films were reduced to be deposited on the surfaces 4 ns of the nickel plating films 4 n of the 60 metal substrates 4 (the surfaces 4 s of the metal substrates 4). Thus, the gold plating films M (the metal plating films) were formed on the surfaces 4 ns of the nickel plating films 4 n of the 60 metal substrates 4 (the film forming step). In this respect, the film formation time and the film formation area of each metal substrate were set similarly to those of Example 1. Consequently, the gold plating films M were able to be formed.

Comparative Example 2

Except that a housing in which a constituent material was changed as follows was used as the housing, the film forming method that formed the gold plating films on the surfaces of the 60 metal substrates was performed similarly to Example 1. Consequently, the gold plating films were able to be formed.

(Housing)

Constituent material: aluminum (A1050)

<<Evaluations for Influence of Constituent Material of Housing and Presence/Absence of Disposition of Porous Film Given to Film Formation>>

In the substitution-type electroless plating method, in a case where the metal plating film is formed on the surface of the metal substrate with the metal substrate brought into contact with a material of a different kind, a current density changes according to an electromotive force of the local cell formed between the metal substrate and the material of the different kind, and this affects the film formation. The current density depends on the kind, the contacted area, and the weight of the material of the different kind with which the metal substrate is brought into contact. In view of this, in the film forming methods of the gold plating films according to Examples and Comparative Examples, the current density changes depending on the kind of the constituent material of the housing with which the metal substrates are brought into contact during the film formation, and this affects the film formation. The following will describe results of the influence of the constituent material of the housing with which the metal substrates are brought into contact during the film formation given to the film formation evaluated together with the influence of presence/absence of the disposition of the porous film in the film forming methods of the gold plating films according to Examples and Comparative Examples.

[Evaluations for Configurations of Gold Plating Films]

The configurations of the gold plating films formed on the surfaces of the nickel plating films of the 60 metal substrates of Example 1 and Comparative Example 1, and Example 2 and Comparative Example 2 were evaluated using a digital microscope (VH-8000 manufactured by KEYENCE CORPORATION). As a result, it has been found that there were many parts where the gold plating films were not formed on the 60 metal substrates in Example 1 and Comparative Example 1 using the housings made of PTFE. On the other hand, the parts where the gold plating films were not formed on the 60 metal substrates were few in Example 2 and Comparative Example 2 using the housings made of aluminum. Furthermore, a comparison between Example 2 and Comparative Example 2 has found that, in Example 2 in which the porous film was disposed on the surfaces of the metal substrates and the float member was disposed on the surface of the porous film, the uniform metal plating film was able to be formed on each metal substrate. Note that as long as the uniform metal plating film can be formed in one process, a yield can be improved and a low cost is possible.

[Evaluations for Weights of Gold Plating Films]

In Example 1 and Comparative Example 1, and Example 2 and Comparative Example 2, the weights of the 60 metal substrates before and after the film formation were measured to evaluate the weights of the gold plating films from the weight changes between before and after the film formation. Furthermore, the averages of the weight changes between before and after the film formation of the 60 metal substrates were calculated to evaluate the average weights of the gold plating films from the averages of the weight changes between before and after the film formation. FIG. 5 is a graph illustrating the averages of the weight changes between before and after the film formation of the 60 metal substrates in Example 1 (the housing made of PTFE is used and the porous film is present) and Comparative Example 1 (the housing made of PTFE is used and the porous film is absent), and Example 2 (the housing made of aluminum is used and the porous film is present) and Comparative Example 2 (the housing made of aluminum is used and the porous film is absent).

As illustrated in FIG. 5, the averages of the weight changes between before and after the film formation in Example 2 and Comparative Example 2 using the housings made of aluminum became larger than the averages of the weight changes between before and after the film formation in Example 1 and Comparative Example 1 using the housings made of PTFE. Furthermore, in a comparison between Example 1 and Comparative Example 1 using the housings made of PTFE, the average of the weight change in Example 1 in which the porous film was disposed became smaller than the average of the weight change between before and after the film formation in Comparative Example 1 in which the porous film was not disposed. In a comparison between Example 2 and Comparative Example 2 using the housings made of aluminum, the average of the weight change in Example 2 in which the porous film was disposed became smaller than the average of the weight change between before and after the film formation of Comparative Example 2 in which the porous film was not disposed.

From the above-described results, it is considered that, in Example 2 and Comparative Example 2 in which the housings made of aluminum were used, the aluminum constituting the housings functioned as the sacrificial anodes to increase the current densities. This accelerated the substitution reaction between the gold and the nickel and increased the weights of the gold plating films. Additionally, it is considered that, in Comparative Example 1 and Comparative Example 2 in which the porous films were not disposed, a large amount of the electroless gold plating solutions were supplied to the surfaces of the nickel plating films of the metal substrates by heat convection. Accordingly, the substitution reaction between the gold and the nickel significantly occurred, and the weights of the gold plating films were increased. Note that when the weight of the metal plating film, such as the gold plating film, increases, since the film forming rate of the metal plating film is improved, mass-production of products using the metal plating films can be achieved in a short period.

[Evaluations for Surface Roughnesses of Gold Plating Films]

In Example 1 and Comparative Example 1 and Example 2 and Comparative Example 2, surface roughnesses Ra of the gold plating films formed on the surfaces of the nickel plating films of the 60 metal substrates were measured from cross-sectional curved lines of the surfaces of the gold plating films using a surface roughness measuring machine (SURFCOM 1400G25 manufactured by TOKYO SEIMITSU CO., LTD.). Furthermore, the averages of the surface roughnesses Ra of the gold plating films of the 60 metal substrates were calculated. FIG. 6 is a graph illustrating the averages of the surface roughnesses Ra of the gold plating films of the 60 metal substrates in Example 1 (the housing made of PTFE is used and the porous film is present) and Comparative Example 1 (the housing made of PTFE is used and the porous film is absent), and Example 2 (the housing made of aluminum is used and the porous film is present) and Comparative Example 2 (the housing made of aluminum is used and the porous film is absent).

As illustrated in FIG. 6, the surface roughnesses Ra of the gold plating films in Example 2 and Comparative Example 2 using the housings made of aluminum had a tendency of being lower than the surface roughnesses Ra of the gold plating films in Example 1 and Comparative Example 1 using the housings made of PTFE. In a comparison between Example 1 and Comparative Example 1 using the housings made of PTFE, the surface roughness Ra of the gold plating film in Example 1 in which the porous film was disposed was lower than the surface roughness Ra of the gold plating film in Comparative Example 1 in which the porous film was not disposed. In a comparison between Example 2 and Comparative Example 2 using the housings made of aluminum, the surface roughness Ra of the gold plating film in Example 2 in which the porous film was disposed was lower than the surface roughness Ra of the gold plating film in Comparative Example 2 in which the porous film was not disposed.

From the above-described results, it is considered that, in Example 2 and Comparative Example 2 in which the housings made of aluminum were used, the aluminum constituting the housings functioned as the sacrificial anodes to increase the current densities. This uniformly caused the substitution reaction between the gold and the nickel and uniformly formed the gold plating film. Additionally, it is considered that, in Comparative Example 1 and Comparative Example 2 in which the porous films were not disposed, a large amount of the electroless gold plating solutions were supplied to the surfaces of the nickel plating films of the metal substrates by heat convection. Accordingly, the substitution reaction between the gold and the nickel did not uniformly occur, and the gold plating films were not uniformly formed. Note that when the surface roughness of the metal plating film, such as the gold plating film, is reduced, since the wettability of the solder to the surface of the metal plating film is improved, the reduction in the number of initial breakdowns of electronic components using the metal plating film can be achieved.

<<Evaluations for Influence of Configurations of Float Members Given to Film Formation>>

The influences of the configurations of the float members given to the film formation by film forming methods of the gold plating films according to Example 2 to Example 7 were evaluated. Specifically, in the film forming methods of the gold plating films according to Example 2 to Example 7, the movements and the actions of the float members in the film forming step were evaluated, and the configurations of the gold plating films formed on the surfaces of the nickel plating films of the 60 metal substrates were evaluated using the digital microscope (VH-8000 manufactured by KEYENCE CORPORATION). From the evaluation results, the influences of the configurations of the float members given to the film formation were evaluated.

In the film forming step in the film forming method according to Example 2, while the float member (the constituent material: PA66, the density: 1.13 g/cm³) was in contact with the side surface inside the housing by the heat convection of the electroless gold plating solution, the float member performed a simple harmonic motion with an amplitude of 0.2 mm in the vertical direction at a position apart from the surface of the porous film by a distance of the amplitude of 0.2 mm in the vertical direction as its oscillation center, and thus the float member repeated the contact with and separation from the surface of the porous film. In association with this, the porous film approached the surfaces of the nickel plating films of the 60 metal substrates without tight contact. This formed a diffusion layer enough to release a gas, such as hydrogen, generated between the metal substrates and the porous film by the deposition of the metal ions. Thus, the uniform gold plating films were formed on the surfaces of the nickel plating films of the 60 metal substrates.

In contrast to this, in the film forming step in the film forming method according to Example 3, despite the heat convection of the electroless gold plating solution, the float member (the constituent material: PTFE, the density: 2.14 g/cm³) was in tight contact with the porous film. The tight contact of the porous film with the surfaces of the nickel plating films of the 60 metal substrates in association with this failed to form the sufficient diffusion layer between the metal substrates and the porous film. Additionally, bubble marks were formed in a part of the gold plating films formed on the surfaces of the nickel plating films of the 60 metal substrates. On the other hand, in the film forming step in the film forming method according to Example 4, the float member (the constituent material: PP, the density: 0.90 g/cm³) floated from the surface of the porous film due to the heat convection of the electroless gold plating solution. Parts where the gold plating films were not formed were generated on the surfaces of the nickel plating films of the 60 metal substrates.

Furthermore, in the film forming steps in the film forming methods according to Examples 5 to 7, while all of the float member of Example 5 (the constituent material: phenolic resin, the density: 1.21 g/cm³), the float member of Example 6 (the constituent material: PET, the density: 1.34 g/cm³), and the float member of Example 7 (the constituent material: PVDC, the density: 1.72 g/cm³) were in contact with the side surfaces inside the housings by the heat convection of the electroless gold plating solutions, the float members performed a simple harmonic motion with amplitudes of 0.2 mm or less in the vertical directions at positions apart from the surfaces of the porous films by distances of the amplitudes of 0.2 mm or less in the vertical directions as their oscillation centers, and thus the float members repeated the contact with and separation from the surfaces of the porous films. In association with this, the porous films approached the surfaces of the nickel plating films of the 60 metal substrates without tight contact. This formed sufficient diffusion layers between the metal substrates and the porous films. Thus, the uniform gold plating films were formed on the surfaces of the nickel plating films of the 60 metal substrates.

The above-described results have proved that the float member having the density of 1.13 g/cm³ or more and 1.72 g/cm³ or less, having the weight larger than that of the porous film, and not having a through-hole is used in some embodiments. It is considered that the absence of the through-hole in the float member uniformly applies a resistance by the electroless gold plating solution on the float member and therefore the float member can perform simple harmonic motion.

While the embodiments of the present disclosure have been described in detail above, the present disclosure is not limited thereto, and can be subjected to various kinds of changes in design without departing from the spirit of the present disclosure described in the claims.

All publications, patents and patent applications cited in the present description are herein incorporated by reference as they are.

DESCRIPTION OF SYMBOLS

-   1 Film forming apparatus -   2 Housing -   2S Housing space -   2 h Opening -   4 Metal substrate -   4 c Copper substrate (copper block substrate) -   4 n Nickel plating film -   6 Porous film -   8 Float member -   10 Lid -   12 Seal -   20 Thermostatic oven -   L Electroless gold plating solution (electroless plating solution) -   L2 Liquid -   M Gold plating film (metal plating film) 

What is claimed is:
 1. A film forming method of a metal plating film for forming a metal plating film on a surface of a metal substrate by solid substitution-type electroless plating method, the film forming method comprising: preparing a film forming apparatus that includes a housing that includes at least a bottom wall and a sidewall surrounding the bottom wall and that is internally provided with a housing space, a metal substrate disposed on the bottom surface inside the housing, a porous film disposed on the surface of the metal substrate, and an electroless plating solution housed in the housing space; and using the film forming apparatus, reducing metal ions derived from the electroless plating solution contained in the porous film, and depositing the metal ions on the surface of the metal substrate to form the metal plating film on the surface of the metal substrate.
 2. The film forming method of the metal plating film according to claim 1, wherein the film forming apparatus further includes a float member disposed on a surface of the porous film, wherein the float member has a density of 1.09 times or more and 1.65 times or less of a density of the electroless plating solution, and wherein the float member has a weight larger than a weight of the porous film.
 3. The film forming method of the metal plating film according to claim 1, wherein the housing is made of a metal serving as a sacrificial anode.
 4. The film forming method of the metal plating film according to claim 3, wherein the film forming apparatus further includes a lid that covers an opening of the housing, and wherein the lid is made of the same material as the material of the housing.
 5. A film forming apparatus for forming a metal plating film on a surface of a metal substrate by solid substitution-type electroless plating method, the film forming apparatus comprising: a housing that includes at least a bottom wall and a sidewall surrounding the bottom wall and that is internally provided with a housing space; a metal substrate disposed on the bottom surface inside the housing; a porous film disposed on the surface of the metal substrate; and an electroless plating solution housed in the housing space.
 6. The film forming apparatus according to claim 5, further comprising a float member disposed on a surface of the porous film, wherein the float member has a density of 1.09 times or more and 1.65 times or less of a density of the electroless plating solution, and wherein the float member has a weight larger than a weight of the porous film.
 7. The film forming apparatus according to claim 5, wherein the housing is made of a metal serving as a sacrificial anode.
 8. The film forming apparatus according to claim 7, further comprising a lid that covers an opening of the housing, wherein the lid is made of the same material as the material of the housing. 